Current cellular wireless networks provide service using large base-stations, referred to as macrocells, that transmit and receive communications channels over relatively large areas. Although the wireless technology is improving at a rapid pace, service providers are exploring the value of introducing a large number of small cellular wireless cells within individual houses, shopping centers and other complexes in order to free capacity for the macrocells and providing better service quality within the small cells. This invention presents a distributed power level selection method in an area that includes many small wireless cells (e.g., femtocells that cover individual houses or even apartments, or picocells that cover larger complexes like shopping centers). The power level selection method must also consider the existence of larger wireless cells (e.g., macrocells) that serve the entire area not covered by the small cells.
The emergence of small cells like femtocells or picocells creates new opportunities and challenges to the cellular carriers. Since these small cells would use the cellular licensed spectrum, they may interfere with the use of macrocells which are the infrastructure used by cellular carriers. Thus, the power levels selected for the femtocells must be coordinated to prevent unacceptable interferences among themselves and for the macrocells. Although the method is generic for small wireless cells introduced within an area served by large wireless cells we use below the terminology of femtocells and macrocells. V. Chandrasekhar, J. G. Andrews, and A. Gatherer, “Femtocell Networks: A Survey”, IEEE Communications Magazine, 46, 59-67, September 2008 and H. Claussen, L. T. W Ho, and L. G. Samuel, “An Overview of the Femtocell Concept”, Bell Labs Technical Journal, 13, No. 1, 221-246, 2008 present overview papers on femtocell networks integrated within an area served by macrocells.
Consider an area with multiple femtocells and macrocells, where the power level of the macrocells is fixed (hereinafter, small cells are referred to as femtocells and large cells as macrocells). The goal is to determine the power level for each of the femtocells so that the following constraints are satisfied:                (i) Each of the femtocells can provide adequate Signal-to-Interference and Noise-Ratio (SINR) throughout the area covered by the femtocell; and        (ii) The macrocells can provide adequate SINK throughout the area served by macrocells.        
It is assumed that a cell-dependent set of critical locations is given as input for each of the femtocells and for each of the macrocells. The critical locations are expected to be those where the SINR is expected to be the worst in the covered area. A viable power level selection method should consider appropriateness of the SINRs at the critical locations associated with each of the femtocells and with each of the macrocells.
Multiple papers have been published on determination of minimum-power solutions. In the context of the present invention, these papers select the minimum power level for each of the femtocells so that their SINR is at least as large as a specified parameter. Although these papers focus on different applications, the issues and the underlying mathematical problems are similar. A sample of these references includes S. V. Hanly, “An Algorithm for Combined Cell-Site Selection and Power Control to Maximize Cellular Spread Spectrum Capacity”, IEEE Journal on Selected Areas in Communications, 13, 1332-1340, 1995, R. D. Yates, “A Framework for Uplink Power Control in Cellular Radio Systems”, IEEE Journal on Selected Areas in Communications, 13, 1341-1347, 1995, and E. Altman and Z., Altman, “S-Modular Games and Power Control in Wireless Networks”, IEEE Transactions on Automatic Control, 48, 839-842, 2003. These references present distributed algorithms that find the minimal powers where all the computations are done locally. These references focus on a single layer of cells, like the femtocells, and do not consider constraints imposed by existing, larger cells such as macrocells.
T. Thanabalasingham, S. V. Hanley, L. L. H. Andrew, and J. Papandriopoulos, “Joint Allocation of Subcarriers and Transmit Powers in a Multiuser OFDM Cellular Network”, IEEE ICC Proceedings, 269-274, 2006 and K. J. Kerpez, T. Lan, K. Sinkar, and L. Kant, “System and Method for Resource Allocation of a LTE Network Integrated with Femtocells”, U.S. Patent Publication No. 2011-0183678, extend the previous work and present algorithms for power allocation in an environment with two types of cells where power levels for both types are decision variables. The fainter reference finds the minimum-power solution, while the latter maximizes data rates while imposing constraints on the selected power levels. Although these references address an important issue of managing two types of cells (lige femtocells and macrocells), an unresolved issue is then to bound the area where power levels are changed for both femtocells and macrocells without affecting other macrocells that are not included in the area. It seems more appropriate to keep the power levels of the macrocells fixed at specific values while determining power levels for femtocells in a bounded area.
The present invention provides a distributed algorithm that determines the maximum SINR that can be satisfied by all the femtocells while also satisfying a specified SINR parameter for the macrocells. The power levels selected for the femtocells are the minimum-power solutions for the maximum SINR that can be satisfied by all the femtocells. The power levels of the macrocells is specified as input and cannot be changed. This algorithm executes all intensive computations independently, locally at each of the femtocells. The timing of the distributed computations is synchronized. The computations are executed simultaneously at all cells where after each of the iterations information of interim power levels selections at the multiple cells is exchanged among the femtocells. Eventually, the distributed computations converge to the maximum SINR value and the corresponding minimum-power solution.